There are electronic watches provided with a built-in electric power generator for displaying time by converting external energy such as photovoltaic energy, thermal energy, mechanical energy, and the like into electric energy (generation of power), and by driving a watch driving system with the use of the electric energy.
Among such electronic watches with the built-in electric power generator, there is a solar cell power generation watch for converting photovoltaic energy into electric energy by use of a solar cell, a mechanical energy conversion power generation watch for converting mechanical energy of the rotation of a rotary weight into electric energy, a temperature difference power generation watch for generating power by utilizing the difference in temperature between the opposite ends of each of integrated thermocouples, and the like.
It is essential for these electronic watches provided with the built-in electric power generator to be driven stably and continuously even in case the supply of external energy is cut off. Accordingly, these electronic watches are also provided with electric power storage means therein such as a secondary cell, a large capacity capacitor, or the like to store electric energy generated by the electric power generator when external energy is available so that the watches can be continuously driven by the electric energy stored in the electric power storage means when the electric power generator is unable to generate power.
This type of electronic watch with a built-in electric power generator has been disclosed in, for example, Japanese Patent Laid-open publication H 4-81754. FIG. 9 is a block diagram showing the general arrangement of the conventional electronic watch with the built-in electric power generator.
In the electronic watch shown in FIG. 9, a small capacitor 132 is connected in series with the electric power generator 130 by external energy, such as a solar cell or the like, via a diode 138 which is first reverse flow preventing means, and clock means 131 and a controller 140 are connected in parallel with the small capacitor 132. Further, a large capacitor 133 is connected in series with the electric power generator 130 via a charge switch 134 and a diode 139 which is second reverse flow preventing means.
A discharge switch 135 is interposed between the small capacitor 132 and the large capacitor 133. Further, first voltage detector 136 and second voltage detector 137 are connected to the small capacitor 132 and the large capacitor 133, respectively, so as to be able to detect terminal voltages of the respective capacitors.
In the electronic watch with the built-in electric power generator when the electric power generator 130 generates sufficient electric power, the clock means 131 is driven by the electric energy supplied thereby while the small capacitor 132 and the large capacitor 133 are charged also by the electric energy. However, when the electric power generator 130 is unable to generate electric power, the clock means 131 is driven continuously by electric energy stored in the capacitors.
When the amount of electric energy stored in the large capacitor 133 is small and the terminal voltage thereof is at a low level and the electric power generator is not generating electric power, the clock means 131 is deactivated owing to the small little amount of electric energy stored in the small capacitor 132, then the charge switch 134 and the discharge switch 135 are open.
When the electronic watch is in this state and the electric power generator 130 starts generation of electric power, electric energy generated is thereby accumulated only in the small capacitor 132, and consequently, the terminal voltage of the small capacitor 132 rises relatively quickly, enabling the clock means 131 to start in a short time after the electric power generator 130 starts to work.
However, even though the clock means 131 is activated by temporary generation of power as described above, there may arise a problem that the clock means 131 comes to a stop in a few seconds after using up a small amount of electric energy stored in the small capacitor 132 when the electric power generator 130 stops generation of power again in a short time.
It happens therefore that when a user attempts to set the electronic watch to right time after checking that a set of watch hands of the clock means 131 has started to move by initial charging of electric energy into the small capacitor 132, the hands comes almost immediately to stop.
Thus, the electronic watch even with the electric power storage means provided therein can not be driven continuously since electric energy stored in the electric power storage means is eventually used up in the case that external energy is not supplied for a long duration.
Even in such a case, the electronic watch can be driven if the electric power generator is caused to start generation of power by resuming supply of the external energy as described above.
In order to accumulate a sufficient amount of electric energy in the electric power storage means which has been once completely depleted to drive the electronic watch continuously, however, it is necessary to either increase the amount of external energy supplied per a unit hour, or prolong the duration of energy supply.
To either increase the amount of external energy supplied per an unit hour, or prolong the duration of energy supply causes a great burden to a user, and consequently, the commercial value of the electronic watch is considerably depreciated.
Furthermore, with some types of electronic watch, depending on the type of electric power generator by converting external energy into electric energy, for example, employing thermoelectric generator, it is impractical to increase the amount of external energy supplied per an unit hour.
Thus, with the conventional electronic watch provided with the built-in electric power generator, recharging the electric power storage means with a sufficient amount of electric energy involves various problems when the electric power storage means is substantially depleted. Since it is not easy to build up a sufficient amount of electric energy when the electric power storage means is substantially depleted as described above, a problem arises that driving of the electronic watch comes to a stop immediately upon stoppage in supply of external energy even for a short time if the electronic watch is activated when the electric power storage means is substantially depleted.
The present invention has been developed to overcome the problems described hereinbefore, and it is an object of the invention to provide an electronic watch with a built-in electric power generator wherein stable driving is effected regardless of variously varying conditions of usage, and particularly, when it is put to use again after being out of use for a relatively long period, starting operation of displaying time can be constantly continued.